Asymmetric-threshold three-terminal switching device

ABSTRACT

An asymmetric-threshold three-terminal electronic switching device includes three terminals coupled to a threshold-switching material. A signal applied across first and second terminals affects an electrical characteristic between the second and third electrodes to a greater extent than the same signal applied across the first and third electrodes. The affected electrical characteristic may be a threshold voltage or conductivity, for example.

CROSS-REFERENCE TO RELATED APPLICATIONS Field of Invention

This invention relates to electronic memory circuits.

BACKGROUND OF THE INVENTION

As electronic memories approach limits beyond which they will no longerbe able to produce the density/cost/performance improvements so famouslyset forth in Moore's law, a host of memory technologies are beinginvestigated as potential replacements for conventional siliconcomplementary metal oxide semiconductor (CMOS) integrated circuitmemories. Among the technologies being investigated are phase changememory technologies. Phase-change memory arrays are based upon memoryelements that switch among two material phases, or gradations thereof,to exhibit corresponding distinct electrical characteristics. Alloys ofelements of group VI of the periodic table, such as Te, S or Se,referred to as chalcogenides or chalcogenic materials, can be usedadvantageously in phase change memory cells. In the chalcogenides, theresistivity varies by two or more orders of magnitude when the materialpasses from the amorphous (more resistive) phase to the crystalline(more conductive) phase, and vice versa. Further, the resistivity of thechalcogenide materials generally depend on the temperature with theamorphous state generally being more temperature dependent that thecrystalline state.

A chalcogenide memory device may utilize the wide range of resistancevalues available for the material as the basis of memory operation. Eachresistance value corresponds to a distinct structural state of thechalcogenide material and one or more of the states can be selected andused to define operational memory states. Chalcogenide materials exhibita crystalline state, or phase, as well as an amorphous state, or phase.Different structural states of a chalcogenide material differ withrespect to the relative proportions of crystalline and amorphous phasein a given volume or region of chalcogenide material. The range ofresistance values is generally bounded by a set state and a reset stateof the chalcogenide material. By convention, the set state is a lowresistance structural state whose electrical properties are primarilycontrolled by the crystalline portion of the chalcogenide material andthe reset state is a high resistance structural state whose electricalproperties are primarily controlled by the amorphous portion of thechalcogenide material.

Phase change may be induced by increasing the temperature locally. Below150° C., both of the phases are stable. Above 200° C., there is a rapidnucleation of the crystallites and, if the material is kept at thecrystallization temperature for a sufficiently long time, it undergoes aphase change and becomes crystalline. To bring the chalcogenide back tothe amorphous state it is necessary to raise the temperature above themelting temperature (approximately 600° C.) and then cool it offrapidly, i.e. quench. From the electrical standpoint, it is possible toreach the crystallization and melting temperatures by causing a currentto flow through a crystalline resistive element that heats thechalcogenic material by the Joule effect.

Each memory state of a chalcogenide memory material corresponds to adistinct range of resistance values and each memory resistance valuerange signifies unique informational content. Operationally, thechalcogenide material can be programmed into a particular memory stateby providing an electric current pulse of an appropriate amplitude andduration to transform the chalcogenide material into the structuralstate having the desired resistance. By controlling the amount of energyprovided to the chalcogenide material, it is possible to control therelative proportions of crystalline and amorphous phase regions within avolume of the material and to thereby control the structural (andcorresponding memory) state of the chalcogenide material to storeinformation.

Each memory state can be programmed by providing the current pulsecharacteristics of the state and each state can be identified, or“read”, in a non-destructive fashion by measuring the resistance.Programming among the different states is fully reversible and thememory devices can be written and read over a virtually unlimited numberof cycles to provide robust and reliable operation. The variableresistance memory functionality of chalcogenide materials is currentlybeing exploited in the OUM (Ovonic Universal (or Unified) Memory)devices that are beginning to appear on the market. Basic principles andoperation of OUM type devices are presented, for example, in U.S. Pat.Nos. 6,859,390; 6,774,387; 6,687,153; and 6,314,014; the disclosures ofwhich are incorporated by reference herein, as well as in severaljournal articles including, “Low Field Amorphous State Resistance andThreshold Voltage Drift in Chalcogenide Materials,” published in EEtransactions on Electron Devices, vol. 51, p. 714-719 (2004) by Pirovanaet al.; and “Morphing Memory,” published in IEEE Spectrum, vol. 167, p.363-364 (2005) by Weiss.

The behavior (including switching, memory, and accumulation) andchemical compositions of chalcogenide materials have been described, forexample, in the following U.S. Pat. Nos. 6,671,710; 6,714,954;6,087,674; 5,166,758; 5,296,716; 5,536,947; 5,596,522; 5,825,046;5,687,112; 5,912,839; and 3,530,441, the disclosures of which are herebyincorporated by reference. These references present proposed mechanismsthat govern the behavior of chalcogenide materials. The references alsodescribe the structural transformations from the crystalline state tothe amorphous state (and vice versa) via a series of partiallycrystalline states in which the relative proportions of crystalline andamorphous regions vary during the operation of electrical and opticalprogramming of chalcogenide materials.

A wide range of chalcogenide compositions has been investigated in aneffort to optimize the performance characteristics of chalcogenicdevices. Chalcogenide materials generally include a chalcogen elementand one or more chemical or structural modifying elements. The chalcogenelement (e.g. Te, Se, S) is selected from column VI of the periodictable and the modifying elements may be selected, for example, fromcolumn III (e.g. Ga, Al, In), column IV (e.g. Si, Ge, Sn), or column V(e.g. P, As, Sb) of the periodic table. The role of modifying elementsincludes providing points of branching or cross-linking between chainscomprising the chalcogen element. Column IV modifiers can function astetracoordinate modifiers that include two coordinate positions within achalcogenide chain and two coordinate positions that permit branching orcrosslinking away from the chalcogenide chain. Column III and Vmodifiers can function as tricoordinate modifiers that include twocoordinate positions within a chalcogenide chain and one coordinateposition that permits branching or crosslinking away from thechalcogenide chain. Embodiments in accordance with the principles of thepresent invention may include binary, ternary, quaternary, and higherorder chalcogenide alloys. Examples of chalcogenide materials aredescribed in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205,5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 thedisclosures of which are all incorporated by reference herein.Chalcogenide materials may be deposited with a reactive sputteringprocess with gasses such as N₂ or O₂ to form a chalcogenide nitride oroxide, for example, and the chalcogenide may be modified by an ionimplantation or other process.

Early work in chalcogenide devices demonstrated electrical switchingbehavior in which switching from an “off” resistive state to an “on”conductive state was induced upon application of a voltage at or abovethe threshold voltage of the active chalcogenide material. This effectis the basis of the Ovonic Threshold Switch (OTS) and remains animportant practical feature of chalcogenide materials. The OTS provideshighly reproducible switching at ultrafast switching speeds. Basicprinciples and operational features of the OTS are presented, forexample, in U.S. Pat. Nos. 3,271,591; 5,543,737; 5,694,146; and5,757,446; the disclosures of which are hereby incorporated byreference, as well as in several journal articles including “ReversibleElectrical Switching Phenomena in Disordered Structures,” PhysicalReview Letters, vol. 21, p. 1450-1453 (1969) by S. R. Ovshinsky;“Amorphous Semiconductors for Switching, Memory, and ImagingApplications,” IEEE Transactions on Electron Devices, vol. ED-20, p.91-105 (1973) by S. R. Ovshinsky and H. Fritzsche; the disclosures ofwhich are hereby incorporated by reference. Three-terminal OTS devicesare disclosed, for example, in U.S. Pat. Nos. 6,969,867 and 6,967,344;the disclosures of which are hereby incorporated by reference.

Although highly efficient and cost effective, process methods and devicestructures that reduce the cost of phase change memories would be highlydesirable.

SUMMARY OF THE INVENTION

An asymmetric-threshold three-terminal electronic switching device inaccordance with the principles of the present invention includes threeterminals coupled to a threshold-switching material. A signal appliedacross first and second terminals affects an electrical characteristicbetween the second and third electrodes to a greater extent than thesame signal applied across the first and third electrodes. The affectedelectrical characteristic may be a threshold voltage or conductivity,for example.

In an illustrative embodiment, an asymmetric-threshold three terminalelectronic switching device in accordance with the principles of thepresent invention is a three-terminal ovonic threshold switch. Byconvention, we refer to one terminal as a control terminal.Structurally, this terminal typically lies between the other twoterminals, with threshold material between itself and each of the otherterminals, and is used to modulate the threshold voltage of the device(that is, the threshold voltage across the other two terminals). Thecontrol terminal forms two terminal-pairs with the other two terminals.One such pair features a lower threshold modulation voltage than theother pair. That is, one pair requires a lesser voltage difference thanthe other pair to reduce the device's threshold voltage to a minimalvalue. By convention, we refer to the terminal in this pair that is notthe control terminal as the reference terminal. Also by convention, werefer to the remaining terminal (that is, the terminal that is neitherthe control nor reference terminal) as the load terminal.

An asymmetric-threshold three-terminal electronic switching device inaccordance with the principles of the present invention may be operated,depending upon circuit connections, much as a p-channel or n-channelenhancement mode MOSFET. For example, with its reference terminalcoupled to a positive supply voltage, its load terminal coupled to anoutput or load node operating at lower or equivalent voltages to thereference supply, and its control terminal connected to receive inputsignals at lower or equivalent voltages to the reference supply, anasymmetric-threshold three-terminal electronic switching device inaccordance with the principles of the present invention may be operatedsimilar to a p-channel enhancement mode MOSFET. Alternatively, couplingthe reference terminal to a negative/common supply voltage, the loadterminal to an output or load node operating at higher or equivalentvoltages to the reference supply, and the control terminal connected toreceive input signals at higher or equivalent voltages to the referencesupply, the device may be operated similar to an n-channel enhancementmode MOSFET.

An asymmetric-threshold three-terminal switching device in accordancewith the principles of the present invention may include ovonicthreshold switching material coupled to three terminals. To produce thedesired threshold modulation asymmetry a greater thickness of thresholdswitching material may be positioned between the control terminal andload terminal than between the control terminal and reference terminal.Such asymmetry may be achieved alternatively or in combination withdifferences in the composition of the threshold switching materialbetween the control/reference and control/load terminal pairs.

An asymmetric-threshold three-terminal electronic switching device inaccordance with the principles of the present invention configured tooperate in a p-channel mode may be referred to herein as “3TP”.Similarly, an asymmetric-threshold three-terminal electronic switchingdevice in accordance with the principles of the present inventionconfigured to operate in an n-channel mode may be referred to herein as“3TN”.

In an illustrative embodiment a 3TP device in combination with a 3TNdevice may be configured to operate as an inverter, with the referenceterminals of the 3TP and 3TN devices respectively coupled to positivesupply and circuit ground, load terminals tied together forming anoutput terminal, and control terminals tied together forming theinverter input. From such a basic circuit, an entire family ofasymmetric threshold three-terminal logic, including NAND gates,flip-flops, latches, and decoders may be produced.

Asymmetric-threshold three terminal electronic switching devices inaccordance with the principles of the present invention may be employedin the peripheral/access circuitry of a memory array, such as a phasechange memory array. In an illustrative embodiment, asymmetric-thresholdthree-terminal electronic switching devices are employed as memory cellaccess devices in such a memory. Additionally, the peripheral circuitryemploys such devices to implement decoding and input/output circuitry.

Such a memory may be particularly suitable for operation in a variety ofelectronic devices, including cellular telephones, radio frequencyidentification devices (RFID), computers (portable and otherwise),location devices (e.g., global positioning system (GPS) devices,particularly those that store and update location-specific information),and handheld electronic devices, including personal digital assistants(PDAs), and entertainment devices, such as MP3 players, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual block diagram of a standalone thin film memory inaccordance with the principles of the present invention;

FIG. 2 is a more detailed conceptual block diagram of a standalone thinfilm memory in accordance with the principles of the present invention;

FIGS. 3A, 3B, and 3C are logic diagrams of thin film decoders,flip-flops, and latches such as may be employed by a standalone thinfilm memory in accordance with the principles of the present invention;

FIG. 4 is a conceptual block diagram of a memory controller andstandalone thin film memory in accordance with the principles of thepresent invention;

FIG. 5 is a conceptual block diagram of a memory controller configuredfor operation with a plurality of standalone thin film memories inaccordance with the principles of the present invention;

FIG. 6 is a more detailed conceptual block diagram of a standalone thinfilm memory controller in accordance with the principles of the presentinvention;

FIG. 7 is a current versus voltage plot for a conventional OTS device;

FIG. 8 is a schematic diagram of a three terminal OTS device inaccordance with the principles of the present invention;

FIG. 9 is a cross sectional view of the structure of a three terminalOTS device in accordance with the principles of the present invention;

FIG. 10 is plot of current versus voltage for a three terminal OTSdevice in accordance with the principles of the present invention;

FIG. 11 is a plot of current versus voltage for asymmetrical 3T OTSdevices in accordance with the principles of the present invention;

FIGS. 12A, 12B, 12C, 12D, and 12E are schematic representations of threeterminal OTS devices in accordance with the principles of the presentinvention;

FIG. 13 is a schematic diagram of a thin film inverter in accordancewith the principles of the present invention;

FIG. 14 is a schematic diagram of a thin film NAND gate in accordancewith the principles of the present invention;

FIGS. 15A through 15L are cross-sectional views of stacked memoryelements in a standalone thin film memory in accordance with theprinciples of the present invention and steps employed in the process offorming such a memory;

FIG. 16 is a top plan view of a standalone thin film memory systemformed on a glass substrate; and

FIG. 17 is a conceptual block diagram of an electronic system such asmay employ standalone thin film memories in accordance with theprinciples of the present invention.

DETAILED DESCRIPTION

Although this invention will be described in terms of certain preferredembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thebenefits and features set forth herein, are also within the scope ofthis invention. Various structural, logical, process step, chemical, andelectrical changes may be made without departing from the spirit orscope of the invention. Polarities and types of devices and supplies maybe substituted in a manner that would be apparent to one of reasonableskill in the art. Although circuits are generally described in terms ofthree terminal ovonic threshold switches (3T OTS), other thin-filmswitching devices, such as amorphous or polycrystalline transistors maybe substituted for the 3T OTS devices in some embodiments. Accordingly,the scope of the invention is defined only by reference to the appendedclaims.

In an illustrative embodiment, a memory in accordance with theprinciples the present invention couples thin-film peripheral circuitrywith thin-film memory to yield a standalone circuit that includes memoryand peripheral circuitry, all of which is rendered using thin filmprocesses and structure. That is, a standalone memory in accordance withthe principles of the present invention is produced by one or more thinfilm processes, such as sputtering or deposition and the resultingstructure of a standalone memory in accordance with the principles ofthe present invention includes thin film layers of polycrystallineand/or amorphous materials formed into thin film memory cells andassociated peripheral circuitry without any single-crystal structures ordevices. Thus the thin film standalone circuits of the present inventiondo not require bulk single-crystal silicon or other elemental orcompound single crystal materials. Additionally, single crystal devicessuch as diodes, MOS transistors, BJT transistors, and SCR devices arenot required to co-exist on the same substrate with the thin filmstandalone circuits. All memory, logic, and other functions on a givenchip are thus performed by thin film devices formed by sequentialdeposition and patterning of thin film materials. Example thin filmmaterials are chalcogenide materials (e.g. GeSbTe 225), interconnectconductive materials (e.g. Al, Cu, W), electrode materials (e.g. C,TiAlN, TiSiN, TiN), and insulators (e.g. SiO₂, SiN_(x), Al₂O₃), and OTSmaterial AsGeInSiTe 35/7/0.25/18/40.

For the purposes of this discussion, a “standalone circuit” refers to anintegrated electronic circuit that is configured to accept at least oneinput from, and provide at least one output to, at least one otherelectronic device. Each standalone circuit typically includes input andoutput structures, such as pads for connection to external circuits andinput and output drivers connected to those pads for communication withcircuits external to the standalone circuit. Various packaging schemesmay be employed with such a standalone circuit, including hybridpackaging, conventional bump bonding, chip-on-board, single-in-linepackaging, dual-in-line packaging, for example. Whatever packagingscheme is employed, the standalone circuit includes input and outputdrivers connected to pads employed for interconnection with otherelectronic circuits.

The conceptual block diagram of FIG. 1 provides a functional level viewof an illustrative standalone thin film memory circuit 100 in accordancewith the principles of the present invention. In this illustrativeembodiment, a standalone thin film memory circuit 100 includes at leastone array of thin-film memory cells arranged as a storage matrix tile102, along with thin-film peripheral circuitry that, in combination,yields the standalone thin-film memory circuit 100. As described ingreater detail in the discussion related to FIG. 2, the thin-film memorycells of the storage matrix 104 may be implemented as, for example,phase change memory cells. In this illustrative embodiment, the memorycircuit 100 includes thin film row drivers 108 and column drivers 106configured to access cells within the storage matrix 104. As describedin greater detail in the discussion related to FIG. 2, the row 108 andcolumn 106 drivers may be, for example, three terminal ovonic thresholdswitches (3T OTS). The three terminals of a 3T OTS device will bereferred to herein as “load”, “control”, and “reference” terminalswhich, as will be described in greater detail in the discussion relatedto FIG. 8, generally correspond, in functional terms, respectively, tothe drain, gate, and source terminals of a field effect transistor. Thinfilm row address, column address and data latches may also be includedin the peripheral circuitry 112.

Accesses carried out by the thin film row 108 and column 106 driversinclude reading from the memory cells of the matrix 104 and writing tothe memory cells of the matrix 104. Peripheral circuitry 112 includesthin-film decoding circuitry 114 which accepts address signals anddecodes the address signals to determine which of the row 108 and column106 drivers to activate and, thereby, which of the memory cells withinthe array 104 to access. Thin film access circuitry 120 including thinfilm current sources for row 108 and column 106 drivers are described ingreater detail in the discussion related to FIG. 2.

In an illustrative embodiment described in greater detail in thediscussion related to FIG. 3, the decoding circuitry 114 may beimplemented using 3T OTS devices. Peripheral circuitry 112 may includecontrol circuitry 116 that employs 3T OTS logic which accepts READ,WRITE, and CLOCK signals and develops control signals for the standalonethin film memory 100. The control signals developed for the memory 100may include data direction control (e.g., “read from” or “write to” thestorage matrix tile 102) and multi-phase clock signals, for example. Inan illustrative embodiment the peripheral circuitry includes 3T OTSinput/output circuitry 118. The input/output circuitry 118 includes 3TOTS circuitry configured to accept data for writing to and to drive dataread from the storage matrix tile 102. Thin film input/output 118 andcontrol circuitry 116 are described in greater detail in the discussionrelated to FIG. 4.

The 3T OTS devices are an example of asymmetric-threshold three-terminalelectronic switching devices in accordance with the principles of thepresent invention, which include three terminals coupled to athreshold-switching material. A signal applied across first and secondterminals affects an electrical characteristic between the second andthird electrodes to a greater extent than the same signal applied acrossthe first and third electrodes. The affected electrical characteristicmay be a threshold voltage or conductivity, for example.

An asymmetric-threshold three-terminal switching device in accordancewith the principles of the present invention may include ovonicthreshold switching material coupled to three terminals. To produce thedesired threshold modulation asymmetry a greater thickness of thresholdswitching material may be positioned between the control terminal andload terminal than between the control terminal and reference terminal.

Such asymmetry may be achieved alternatively or in combination withdifferences in the composition of the threshold switching materialbetween the control/reference and control/load terminal pairs. Notwishing to be bound by theory, it is believed that filament formation inthreshold switching material is enhanced by the use of materials thatexhibit lower binding energies in their valence electrons. In anillustrative embodiment the threshold material between control andreference terminals may be tellurium-rich, for example, to reduce themodulation threshold voltage for the control/reference terminal pair.The threshold material between control and load terminals may be sulfur-or selenium-rich to raise the modulation-threshold voltage for thecontrol/load terminal pair.

The conceptual block diagram of FIG. 2 provides a more detailed view ofa portion of a standalone thin-film memory in accordance with theprinciples of the present invention. The illustrative memory segment 200includes access current sources labeled READ, WRITE0, and WRITE1, all ofwhich are implemented with thin film devices. Although the currentsources may be implemented using thin-film transistors, such aspolycrystalline or amorphous transistors; 3T OTS devices, labeled 3TCUR, are employed in this illustrative embodiment.

In this illustrative embodiment the load terminals of 3T CUR devices arecoupled to a positive supply source, control signals READ, WRITE0, andWRITE1 are directed to their respective control terminals and all theirreference terminals are coupled to a common node 202. Each of thecurrent supply devices 3T CUR is biased to provide a current pulseappropriate for its selected operation to the common node 202. That is,when one of the control signals READ, WRITE0, or WRITE1 is activated,the corresponding 3T OTS directs, respectively, a READ, WRITE0, orWRITE1 current pulse to the common node 202.

In this illustrative embodiment, each memory cell includes a phasechange memory, labeled OUM, which is coupled to a return path on oneside, as indicated by the connection to “ground” in the figure. Theother side of each phase change memory is coupled to a referenceterminal of a 3T OTS labeled 3T CELL. The control terminals of the 3TCELL devices are coupled to a row access signal so that the controlterminals of all 3T OTS devices in a row are coupled to the same rowaccess signal (e.g. ROW1, ROW2, . . . ROWn). Similarly the loadterminals of all 3T OTS devices in a given column are coupled to thesame column access signal (e.g., COL1, COL2, . . . COL3) through 3T OTSdevices labeled 3T COL. Each memory device OUM may thereby be accessedindividually by asserting a corresponding row/column pair of accessdevices. For example, cell n,m may be accessed by asserting the row nsignal (coupled to the control terminal of 3T OTS n,m) and the column msignal (coupled to the load terminal of 3T OTS n,m through the referenceterminal of 3T COL device m).

In this illustrative embodiment, the load terminals of the 3T COLdevices are all coupled to common node 202 through which access currentsare directed from READ, WRITE0, and WRITE1 current sources, aspreviously described. In this way, the column access 3T OTS devices 3TCOL operate to “gate” the appropriate access current to a selectedcolumn of devices and the row access 3T OTS devices operate to provide areturn path for the current pulse through a selected one of the OUMmemory devices coupled to the “gating” column access device 3T OTS. Inthe case of a WRITE access, the current pulse operates to program aselected device to a desired memory state (although only two states, oand 1, are illustrated, multiple states are contemplated within thescope of the invention). In the case of a READ access, the current pulseoperates to establish a voltage across a selected OUM device, which issensed through the low impedance path of the selected column accessdevice 3T COL at the column node 202. The READ voltage appearing at thecommon node 202 may be sensed, as indicated by the arrow labeled SENSE,by a sense amplifier (not shown) and converted to a digital data valueby one or more comparators (not shown), for example. As described ingreater detail in the discussion related to FIG. 4, the analog sensevalue may be sent “off-chip” for digitization. Additionally, astandalone thin-film memory in accordance with the principles of thepresent invention may include circuitry configured to receive one ormore reference voltages or current pulses from “off-chip.” Suchreference voltages or currents may be employed, for example, in thegeneration of access currents used to write to or read from OUM deviceswithin the memory segment 200. In an illustrative embodiment an externalmemory controller provides current pulses used to access standalone thinfilm memories in accordance with the principles of the presentinvention. Providing current pulses in this manner ensures that the 3TOTS devices used as access devices turn off after they've been used inan access operation.

The conceptual block diagrams of FIGS. 3A, 3B, and 3C illustrates thebasic organization of a thin film decoder 300, a flip-flop 302, andinverting latch 304 in accordance with the principles of the presentinvention. Such a decoder may be included in the peripheral circuitry112 of FIG. 1 to accept address signals and to provide row and columndrive signals to row drivers 108 and column drivers 106, and, moreparticularly, to activate row 3T CELL and column 3T COL 3T OTS driversdescribed in the discussion related to FIG. 2. The disclosed thin-filmlogic functions may be implemented using 3T OTS devices, as described ingreater detail in the discussion related to FIGS. 7-14.

In this illustrative embodiment, the decoder 300 receives address linesA0 and A1 and, from the address signals, derives row access signalsROW1, ROW2, ROW3, and ROW4. Address line A0 is coupled to a first inputof two-input NAND gate NAND4, to the input of an inverter INV2, and to afirst input of NAND gate NAND2. The inverted address A0 input (output ofinverter INV2) is connected to a first input of NAND gate NAND3 and afirst input of NAND gate NAND1. Address input A1 is connected to secondinputs of NAND gates NAND3 and NAND4, and to the input of inverter INV1.The inverted address A1 input (output of inverter INV1) is connected tothe second inputs of NAND gates NAND1 and NAND2.

Increasing address line signal values provide positive outputs andincrementing row line outputs. That is; for address line A0, A1 value0,0, the Row1 line is asserted (brought low), for address line A0, A1value 0,1, the Row2 line is asserted, for address line A0, A1 value 1,0,the Row3 line is asserted, and for address line A0, A1 value 1,1, theRow4 line is asserted. A decoder 300 in accordance with the principlesof the invention may be extended to any number of address inputs and rowselection outputs. Additionally, a decoder 300 in accordance with theprinciples of the present invention may also be used to decode addressesfor column selection outputs.

The flip-flop 302 of FIG. 3B employs two cross-coupled thin film NANDgates, NAND1 and NAND2, such as described in greater detail in thediscussion related to FIGS. 7-14. Gate NAND1 receives a SETbar input atone of its input terminals and the output of gate NAND2 at its otherinput terminal. Gate NAND2 receives a RESETbar input at one of its inputterminals and the output of NAND1 at its other input terminal. Theoutputs of gates NAND1 and NAND2 are Q and Qbar. Thin film flip-flopssuch as this may be extended to any desirable width and used, forexample to latch row and column addresses into row and column decoders,for example.

The inverter latch 304 of FIG. 3C includes two cross-coupled inverters,INV1 and INV2. In an illustrative embodiment, each of the inverters,INV1 and INV2, is a three-terminal OTS-based inverter, such as theinverter described in greater detail in the discussion related to FIG.13. In operation, a logic “1” input generates a logic “0” output. Thelogic “0” output is fed to the input of INV2, which generates a logic“1” output. The logic “1” output of INV2 is fed back to the input ofINV1, which reinforces the original logic “1” input, thereby creating astable, latching circuit. If the input value is changed to a logic “0,”the output of inverter INV1 changes to a logic “1,” which propagates tothe input of inverter INV2. The logic “1” at the input of inverter INV2generates a logic “0” which is fed back to the input, IN, of inverterINV1, another stable state for the latch 304. A plurality of latches,such as latch 304 may be combined to create a latch of a desired width:the width of a data (or address) path, for example. As described ingreater detail in the discussion related to FIG. 6, such latches may beused to store test values and/or test modes within a standalone thinfilm memory in accordance with the principles of the present invention.

FIG. 4 is a conceptual block diagram of a thin film input/output andcontrol (I/O) interface 400 such as may be employed by a standalone thinfilm memory in accordance with the principles of the present invention.In this illustrative embodiment a sense signal SENSE is sent “off-chip”to a sense and compare circuit located on a chip other than thestandalone thin film memory. The sense and compare circuit 402 acceptsthe SENSE signal and converts it from an analog signal to a digitalsignal, thereby developing a digital data output signal(s) for a memoryin accordance with the principles of the present invention. The senseand compare circuit 402 may be included, for example, in a memorycontroller circuit, such as is described in greater detail in thediscussion related to FIG. 6.

The SENSE signal, which is indicative of an accessed memory cell'sprogrammed level (manifested, for example, as a voltage level for a cellthat exhibits different values of resistance according to its programmedlevel and to which a current source is delivered for sensing), may betaken from a node, such as node 202 of FIG. 2, “directly atop” anarray's column line. In an illustrative embodiment, with a 3T OTS columndriver turned “ON” in order to access a cell and a current supplied toan OUM cell by a 3T OTS activated by a READ signal, the voltage at node202, which is also the SENSE voltage, is approximately the product ofthe current through the OUM multiplied by the resistance of the OUM plusan offset voltage attributable to the 3T OTS column driver and 3T Cellaccess device.

Data lines DATA0-DATAP, address lines ADDRESS0-ADDRESSL, and READ andWRITE control signals are received by the I/O circuit 400 and convertedto WRITE0, WRITE1, READ, COL0-COLm, and ROW0-ROWn signals. The WRITE0,WRITE1, READ, COL0-COLm, and ROW0-ROWn signal functions have beendescribed in the discussion related to FIG. 2. Although only two-levelstorage, corresponding to WRITE0 and WRITE1, is employed in thisillustrative embodiment, a thin film memory in accordance with theprinciple of the present invention may employ a scheme whereby eachmemory cell within a thin film memory array is capable of taking on anyone of more than two prescribed levels (resistance or voltage levels,for example), corresponding to a data content of more than one bit percell.

The I/O circuit 400 also receives a clock input from which it developsinternal timing and control signals. In an illustrative embodiment, theI/O circuit develops a multi-phase internal clock signal, labeled INTCLOCK, from a received single-phase system clock. A multi-phase clockand its operation in a thin-film memory in accordance with theprinciples of the present invention will be described in greater detailin the discussion related to FIGS. 7-14. The internal clock INT CLOCKmay also include one or more signals that control the latching of dataor intermediate results, for example. A control input ENABLE may beemployed by external circuitry to enable the thin film memory in aconventional fashion, for example, to enable the phase change memoryafter a power-on-reset sequence has been completed.

The I/O circuit 400 may also include test mode support that may operatein conjunction with a memory controller to test operation of thestandalone thin film memory. Such testing may include built in selftests. Built in self tests are known and described, for example, in“COMPREHENSIVE STUDY ON DESIGNING MEMORY BIST: ALGORITHMS,IMPLEMENTATIONS AND TRADE-OFFS,” by Allen C. Cheng, Department ofElectrical Engineering and Computer Science, The University of Michigan,which is hereby incorporated by reference.

In accordance with the principles of the present invention, the I/Ocircuit 400, located on a standalone thin film memory 100, may includecircuitry that allows a standalone thin film memory 100 in accordancewith the principles of the present invention to off-load functionalityto external circuitry, such as an external controller, for example. Inan illustrative embodiment, the I/O circuit 400 includes a test register404, a reconfiguration register 406, and a test mode register 408. Astandalone thin film memory 100 in accordance with the principles of thepresent invention may also include test pads that permit externalcircuits to probe and exercise components of the memory 100. Suchcircuitry allows a standalone thin film memory in accordance with theprinciples of the present invention to be configured for operation withan external controller. For example, the test register 404 may beconfigured to receive input from an external device, such as patternsthat may be used to “exercise” memory elements within the memory arrayand to thereby uncover faulty elements. Results of theexercising/testing process may be evaluated in an external circuit, suchas a memory controller in accordance with the principles of the presentinvention that is configured for operation with standalone thin-filmmemories in accordance with the principles of the present invention. Thereconfiguration register 406 may be devised to operate in conjunctionwith an external controller to reconfigure portions of the memory arrayin response to test results. That is, for example, an externalcontroller may identify a fault within a segment of the memory array100, in response, provide memory mapping information to thereconfiguration register 406 that allows the reconfiguration register tomap redundant memory within the memory array 100 into the “active”memory-mapped space of the memory 100 and to map the faulty memorysegment out of the active memory-mapped space.

In accordance with the principles of the present invention, a test moderegister 408 includes test mode instructions that may be used by thememory 100 to implement one or more test modes. For example, astandalone thin film memory 100 may cycle through memory locations usinga nested do-loop of column, then row (or, row then column) addresses toimplement a memory test. In accordance with the principles of thepresent invention, an all-thin-film counter, composed of thin film logiccircuits such as the inverters, flip-flops, and latches described in thediscussion related to FIGS. 3A through 3C, may be used to increment therow and column addresses employed within the nested do loop. In thismanner, an external controller may initiate a memory test without beingburdened with the task of updating addresses and test patterns to cellswithin the memory 100. Test patterns employed by the memory 100 mayinclude: “all 1s,” “all 0s,” checkerboard, and checkerboard complement,for example, and a test code stored within the test mode register 408may determine which of the test patterns to employ.

The on-chip address-generation circuitry may also be employed toimplement a self-refresh mode, whereby the memory 100 cycles through ado loop of column, then row (or row, then column) addresses and readsthen rewrites (with the same data) the active cells within the array100. Such a self-refreshing process may be initiated, by a memorycontroller, for example, at strategic times, such as during system powerup or power down. An all thin film memory 100 in accordance with theprinciples of the present invention may include input protectioncircuits, high speed data interface circuits for communications with anexternal controller, address transition detection circuits for operationwith an external controller and user-selectable operating mode detectioncircuits for operation with an external controller.

The conceptual block diagram of FIG. 5 illustrates a memory system 500in accordance with the principles of the present invention. The memorysystem 500 includes at least one standalone thin film memory 100 inaccordance with the principles of the present invention, such asdescribed in greater detail in the discussion related to previousfigures. A standalone thin film memory controller 502 in accordance withthe principles of the present invention is connected through DATA 504,ADDRESS 506, and CONTROL 508 busses to the one or more thin filmstandalone memories 100.

In this illustrative embodiment the thin film standalone memorycontroller 502 includes at least one sense amplifier and encoder 510configured to receive analog data from the thin film memory 100 toconvert it from analog to digital form and to encode the datarepresented by the analog signal. The sense/encode circuit 510 assignsdifferent binary values, that is, encodes differently, an analog signalreceived from the memory 100, depending upon whether the memory 100 isconfigured as a binary, quaternary, or other “n-ary” memory. That is,the encoding process reflects the fact that an analog voltage signalcorresponding to a given resistance within a memory cell (such as a cellOUM of FIG. 2) may be assigned one digital value for a binary memoryoperation but a different digital value for a quaternary memoryoperation, for example.

In an illustrative embodiment, the memory controller 502, through itscontrol circuit 512, coordinates access to analog data from eachstandalone memory 100. Each standalone thin film memory 100 may providemultiple analog outputs, one for each data line, for example, and thememory controller 502 may accommodate the multiple analog lines byemploying an analog multiplexer to feed analog signals from a number ofanalog data lines to a lesser number of sense amplifier/encoders.Alternatively, the controller 502 may match the number of senseamplifier/encoders to the number of analog data lines from thestandalone thin film memory 100, which may be equal to the number ofdata lines, or the controller 502 may provide more senseamplifier/encoders than the number of analog data lines in order toexpedite the analog-to-digital conversion and encoding processes.

Encoded digital data from the sense amplifier/encoder circuit 510 isprovided to the data block 514 which provides an interface with anexternal data bus 516, such as data bus 1730 described in greater detailin the discussion related to FIG. 17. The data block 514 may includebuffers and other associated circuitry configured to match the data rateof the one or more memories 100 to the data rate of the external databus 516.

The control circuit 512 may include one or more output enable signalsthat control access to the analog bus 509 in a manner that preventscontention. In this illustrative embodiment, the output enable signal(s)are among the previously described enable signal(s) provided to astandalone memory 100. Address circuitry 518 provides an interfacebetween an external address bus 520 and the memory address bus 506. Theaddress circuitry 518 translates addresses presented on the externaladdress bus 520 into addresses appropriate for each of the standalonethin-film memories 100 within the memory system 500. Control circuitry512 provides an interface with an external control bus 522 and, in thatcapacity, develops control signals, including clock and enable signalsfor delivery to standalone thin film memories 100 through the memorycontrol bus 509.

Functions, and associated circuits, that might otherwise be performed oneach of the standalone thin film memories 100 may be concentrated on thememory controller 502, thereby reducing the cost of production for eachof the memories 100, while only incrementally increasing the cost andcomplexity of the controller 502. The memory controller 502 may itselfbe an all-thin film device or it may be produced using a moreconventional technology, such as complementary metal oxide semiconductor(CMOS) technology.

The conceptual block diagram of FIG. 6 depicts a memory controller 600in accordance with the principles of the present invention. The memorycontroller 600 is adapted for operation with standalone thin filmmemories in accordance with the principles of the present invention and,as such, includes functional elements that allow each standalone thinfilm memory to offload functionality to the controller 600 and therebyreduce the complexity and cost of the standalone thin film memoriesserviced by the controller 600.

The controller 600 includes system data 602, system address 604, andsystem timing and control 606 bus interfaces. Control logic 608 directsexecution of the memory controller 600. A write buffer 610 operates astemporary storage for data that is to be written to memory (such as oneor more standalone thin film memories 100) through a data interface 612.An analog interface 614 accepts analog input from one or more standalonethin film memories. In an illustrative embodiment, each analog inputvalue is an analog voltage that is representative of the resistance of aselected memory device, such as one of the memory devices labeled OUM inFIG. 2. The analog interface 614 may include signal conditioning orbuffering circuitry, for example. Analog signals received at the analoginterface 614 are sensed and encoded by sense/encoding circuitry 616. Aspreviously noted the encoding circuitry may assign different digitalvalues to a given analog signal, depending upon whether the memory fromwhich the signal is obtained is a binary or “other-ary” memory.

In an illustrative embodiment, encoded digital information from thesensing/encoding circuit 616 is transferred to a read buffer 618 whichprovides temporary storage for data read from a memory. The controllogic 608 operates the read 618 and write 610 buffers to executetransfers of data between the system and memory sides of the controller600. Such transfers may include direct memory access or double data ratetransfers, for example. Control logic 608 exercises control over address620 and timing and control circuitry 622 to implement such datatransfers.

In an illustrative embodiment the memory address interface 620 operatesby providing addresses to standalone thin film memories in accordancewith the principles of the present invention by multiplexing addresses(that is, by sequentially presenting “row” and “column” addresses). Suchaddress multiplexing is known in the art and permits a reduction in thenumber of physical connections, referred to as “pinout,” required forthe address interface 620.

The memory timing and control circuit 622 produces signals, Row AddressStrobe (RAS) and Column Address Strobe (CAS), that operate to latch therow and column components of addresses into memories being accessed.Additionally, the memory timing and control circuit 622 provides READand WRITE signals to the memories being controlled by the memorycontroller 600, which the memories employ, as previously described, todevelop the appropriate signal (READ, WRITE1, or WRITE0 current pulse,for example) for a memory cell being accessed. In an illustrativeembodiment, the memory timing and control circuit 622 generates one ormore chip select signals, CHIP SELECT, which may be employed by thecontroller 600 in the addressing/memory-mapping scheme.

One or more clock signals, CLOCK, may be distributed to the standalonethin film memories under control of the controller 600. Such clocksignals may be used by the standalone thin film memories to developmultiphase clocks for the execution of internal logic or may be used tosynchronize the memories with a system clock in order to permit rapidtransfers of data between the system and memory, for example.

Test signals, labeled TEST, generated by the memory timing and controlcircuit 622 are employed by standalone thin film memories to performself tests. As described in the discussion related to FIG. 14 eachstandalone thin film memory may include self-test circuitry. In anillustrative embodiment, a memory controller in accordance with theprinciples of the present invention 600 may provide support for suchself tests through use of the TEST signal(s). The memory timing andcontrol circuit 622 may include circuits for memory sequencing, sensingcircuits, I/O signal amplification, redundancy control circuits, delayelements, test mode control circuits, reliability stress algorithms,address transition detection circuits (from the system address bus),user selectable operating mode detection circuits (from the system),voltage and/or current reference generators, or voltage generatormodules. The memory timing and control circuit 622 may also includecircuitry configured to operate with control logic 608 and the systembus interface (602, 604, 606) to provide a high speed data interface,for example.

In an illustrative embodiment, the memory controller 600 may beconfigured to provide refresh control for standalone thin-filmprogrammable resistance memories in accordance with the principles ofthe present invention. That is, a standalone thin-film memory inaccordance with the principles of the present invention may exhibit awide range of data retention characteristics, varying from microsecondsto decades and for those memories that exhibit relatively shortretention periods, a memory controller in accordance with the principlesof the present invention includes refresh circuitry that ensures thatthe memory retains data for a period of time appropriate to a givenapplication. For example, the memory timing and control circuit 622 mayoperate the various memory interfaces (including the memory, data, andanalog interfaces, for example) to perform a periodic read/rewrite cycleon each memory location in the bank of standalone thin film memoriesunder control of the memory controller 600.

The memory controller 600 may also include circuitry configured foridentifying and correcting “weak bits” within a standalone memory arrayin accordance with the principles of the present invention. The term“weak bits” refers to memory cells or elements that, although they havenot failed, hold values that fall outside a preferred range of valuesand provide less than a desirable margin for read. To that end, in anillustrative embodiment the controller 600 may include circuitryconfigured to sense the value of a memory cell (which value may take theform of a resistance value) and compare that value to a preferred rangeof values. If the value stored within a memory falls outside thepreferred range, but is, nevertheless determined to be valid (indicatingthat the cell has not failed), the controller rewrites the cell to avalue within the preferred range.

As previously described, a 3T OTS may be employed as an access devicewithin a thin film memory array (e.g. 3T COL, 3T CELL of FIG. 2) or as acomponent in a logic device, such as the decoder 300 of FIG. 3. Threeterminal ovonic threshold switches are known in the art. Early work inchalcogenide devices demonstrated electrical switching behavior in whichtwo terminal (2T) OTS switching from an “off” resistive state to an “on”conductive state was induced upon application of a voltage at or abovethe threshold voltage of the active chalcogenide material. This effectis the basis of the Ovonic Threshold Switch (OTS) and remains animportant practical feature of chalcogenide materials. The OTS provideshighly reproducible switching at ultrafast switching speeds. Basicprinciples and operational features of the OTS are presented, forexample, in U.S. Pat. Nos. 3,271,591; 5,543,737; 5,694,146; and5,757,446; the disclosures of which are hereby incorporated byreference, as well as in several journal articles including “ReversibleElectrical Switching Phenomena in Disordered Structures,” PhysicalReview Letters, vol. 21, p. 1450-1453 (1969) by S. R. Ovshinsky;“Amorphous Semiconductors for Switching, Memory, and ImagingApplications,” IEEE Transactions on Electron Devices, vol. ED-20, p.91-105 (1973) by S. R. Ovshinsky and H. Fritzsche; the disclosures ofwhich are hereby incorporated by reference. Three-terminal OTS devicesare disclosed, for example, in U.S. Pat. Nos. 6,969,867 and 6,967,344;the disclosures of which are hereby incorporated by reference. A logicsystem employing three terminal OTS devices is disclosed in a patentapplication filed Mar. 15, 2007, entitled, “Multi-terminal ChalcogenideLogic Circuits,” and having Ser. No. 11/724,485 which is herebyincorporated by reference.

The switching properties of a 3T OTS are depicted in the current vs.voltage plot of FIG. 7. The illustration of FIG. 7 corresponds to atwo-terminal device configuration in which two spacedly disposedelectrodes are in contact with a chalcogenide material and the current Icorresponds to the current passing between the two electrodes. The I-Vcurve of FIG. 7 shows the current passing through the chalcogenidematerial as a function of the voltage applied across the material by theelectrodes. The I-V characteristics of the material are symmetric withrespect to the polarity of the applied voltage. For convenience, weconsider the first quadrant of the I-V plot of FIG. 7 (the portion inwhich current and voltage are both positive) in the brief discussion ofchalcogenide switching behavior that immediately follows. An analogousdescription that accounts for polarity applies to the third quadrant ofthe I-V plot, for a symmetrical device.

The I-V curve includes a resistive branch and a conductive branch, asindicated in FIG. 7. The resistive branch corresponds to the branch inwhich the current passing through the material increases only slightlyupon increasing the voltage applied across the chalcogenide material.This branch exhibits a small slope; it appears as a nearly horizontalline. The conductive branch corresponds to the branch in which thecurrent passing through the material increases significantly uponincreasing the voltage applied across the chalcogenide material. Thisbranch exhibits a large slope; it appears as a nearly vertical line. Theslopes of the resistive and conductive branches shown in FIG. 7 areillustrative and not intended to be limiting, the actual slopes willdepend on the detailed characteristics of an actual device, includingthe resistance of contacts and the chemical composition of thechalcogenide material.

When no voltage is applied across the chalcogenide material, thematerial is in a resistive state and no current flows. This conditioncorresponds to the origin of the I-V plot shown in FIG. 7. Thechalcogenide remains in a resistive state as the applied voltage isincreased, up to a threshold voltage V_(t). The slope of the I-V curvefor applied voltages between 0 and V_(t) is small, reflecting the highresistance of the chalcogenide material. This high resistance state maybe referred to, particularly in digital applications, as the OFF stateof a chalcogenide device. When the applied voltage equals or exceedsV_(t), the chalcogenide material quickly switches from a highlyresistive state to a highly conductive state, as indicated by the dashedline in FIG. 7. The chalcogenide material remains in the conductivebranch as long as a minimum current I_(h) referred to as the device'sholding current, is maintained.

By adding one or more terminals to a chalcogenide switching devicehaving the I-V characteristics displayed in FIG. 7, the threshold levelof such a device may be manipulated. Multi-terminal chalcogenideswitching devices are known and described, for example, in U.S. Pat.Nos. 6,967,344 and 6,969,867 and an application having publicationnumber 2004/0178401, which are hereby incorporated by reference. In theschematic diagram of FIG. 8, a 3T OTS 800 device includes thresholdswitching material 802, such as chalcogenide material, a load terminal804, a control terminal 806, and a reference terminal 808. By adding acontrol terminal 806 (and associated structure) to an OTS, a 3T OTSallows for the modulation of an OTS device's threshold voltage, therebyenabling controlled switching of the device 800.

In the absence of a control signal, the chalcogenide material switchesfrom a resistive state to a conductive state upon application of athreshold voltage, where the magnitude of the threshold voltagecorresponds to the threshold voltage between load 804 and reference 808terminals. A suitable control signal at the control terminal 806influences the threshold switching voltage of the chalcogenide material802 between the load 804 and reference 808 terminals. With no voltagebetween the control 806 and reference 808 terminals, and a voltageapplied across the load 804 and reference 808 terminals that is lessthan the threshold voltage of the device 800, the device 800 will remainin a high resistance, non-conducting, “OFF” state. By applying a voltageor current to the control terminal 806 (that is, a voltage measuredbetween the control 806 and reference 808 terminals), the thresholdvoltage across the load 804 and reference 808 terminals will be reduced.With proper biasing (e.g., with sufficient voltages or currents appliedto the control and load terminals), the device 800 will switch to ahighly conductive “ON” state.

FIG. 9 is an illustrative cross section of a 3T OTS, such as may beemployed in a device in accordance with the principles of the presentinvention. The three terminals are labeled T(1), T(2), and T(3). Such adevice may be formed using conventional sputtering, chemical vapordeposition, etching, and lithography techniques. In this illustrativeembodiment, the structure includes a substrate 910, a thermal oxidelayer 920, a bottom electrode 930 that includes a conductive layer 940formed from TiW or a combination of Ti and TiN and a carbon barrierlayer 950, an SiO_(x) or SiN_(x) insulating region 960, a controlelectrode 970 formed from TiW, a chalcogenide material 980, a topelectrode 990 that includes a carbon barrier layer 900 and a conductivelayer 912 that includes Ti and TiN, and an Al layer 922.

In this example, the chalcogenide material 980 is an OTS material, asfor example an alloy composed of AsGeInSiTe 35/7/0.25/18/40 and islabeled OTS in FIG. 9. The barrier layers inhibit diffusion andelectromigration of material into the chalcogenide region and improvethe cycle life of the device. Typical layer thicknesses are as follows:conductive layer 940 (100 nm), barrier layer 950 (30 nm), controlelectrode 970 (10-40 nm), barrier layer 900 (100 nm), and conductivelayer 912 (100 nm). The region occupied by the chalcogenide material inthe device of this example is cylindrical with a height of approximately0.1 micron and a diameter of about 1 micron. The region occupied by thechalcogenide material in this illustrative embodiment may be referred toherein as a pore or pore region for example.

The electrodes 930, 970 and 990 are in electrical communication with thechalcogenide and correspond to terminals 808, 806, and 804,respectively, of FIG. 8. The control electrode 970 circumscribes thechalcogenide material 980. As previously noted, the top electrode 990and bottom electrode 930 may also be referred to as the load andreference electrodes, respectively. In circuits analogous tocommon-emitter configurations, the reference electrode 930 may beconnected in the “common” or “reference” position. The electrodes areseparated by insulating material so that electrical communicationbetween electrodes occurs through the chalcogenide material.

In an illustrative embodiment, the substrate 910 may be an amorphousmaterial, such as glass. Such an embodiment will be described in greaterdetail in the discussion related to FIG. 15. The composition and/orthickness of the various layers, and, in particular, of the chalcogenidelayer, may be tailored to adjust operational parameters of the device.Such tailoring may include tailoring of 3T OTS for preferentialoperation in the I or III quadrant of the I-V plane, for example.

The graphical representation of FIG. 10 depicts the first quadrant of anI-V plot which illustrates the switching characteristics of a 3T OTSsuch as may be used in a device in accordance with the principles of thepresent invention. The current I corresponds to the current passingbetween the load (top) and reference (bottom) electrodes of thestructure and the voltage V corresponds to the voltage applied betweenthe load and reference electrodes. In this illustrative embodiment, theI-V relationship between the load and reference electrodes are plottedfor several different control voltages applied to the control electrode.The plots represent an operation in which a control voltage of constantmagnitude is applied to the control terminal and the current between theload and reference electrodes measured as a function of the voltageapplied between the load and reference electrodes. In this illustrativeembodiment, the control voltage may be applied in the form of a longduration voltage pulse (e.g. 3 microseconds) and the voltage between theload and reference electrodes applied in the form of a short durationpulse (e.g. 100 nanoseconds) while the control voltage was beingapplied. In this example, the control voltage is applied between thecontrol electrode and reference electrode of the device.

The data in FIG. 10 indicate that application of a control voltage tothe control electrode may be used to modulate the threshold voltagebetween the load and reference electrodes. The different I-V curvescorrespond to tests using different control voltages. The controlvoltage associated with each I-V curve is indicated with a label in FIG.10. The I-V curve labeled “0-2V” shows the behavior of the device forcontrol voltages between 0 V and 2 V, inclusive. Since the I-Vcharacteristics of the device are substantially identical for controlvoltages in this range, a single curve is presented for applied voltagesin this range. The “0-2V” data indicate that the resistive branch of theI-V curve extends from an applied voltage of 0 V up to a voltagethreshold voltage of about 1.56 V. Once the threshold voltage isreached, the device switches to the conductive branch. The switchingtransformation is indicated by the negative sloped line segments in theI-V curve.

In this illustrative embodiment, increasing the control voltage above 2V yields a decrease in the device's threshold voltage. The I-V curvelabeled “2.5V” indicates that a control voltage of 2.5 V reduces thethreshold voltage by over 10% to a value slightly below 1.4 V. A furtherincrease of the control voltage to 3 V leads to a decrease in thethreshold voltage of about 25% to a value of about 1.2 V. When a controlvoltage of 4 V is applied, the threshold voltage is effectivelyeliminated and the chalcogenide material between the load and referenceelectrodes is in its conductive state over the full range of appliedvoltages (that is, voltages between the load and reference terminals).

The data presented in FIG. 10 demonstrate an ability to modulate thethreshold voltage between two electrodes of a multi-terminal device byapplying a control voltage to a control terminal. The modulation effectrepresents functionality achievable in the instant multi-terminaldevices that is not available in standard two-terminal devices. Whilenot wishing to be bound by theory, it is believed that application of acontrol signal of sufficient magnitude to a control terminal mayfacilitate formation of a conductive filament between the controlterminal and another of the 3T OTS terminals, thereby producing theobserved reduction in threshold voltage between load and referenceterminals.

The minimum control signal required to facilitate filament formation maybe referred to as a critical control signal (between 2.0V and 2.5V forthe example discussed in relation to FIG. 10). It is believed that thecritical control signal is the minimum signal required to form afilament within the chalcogenide material between the control terminaland another of the terminals (that is, reference or load terminal). Thepresence of this filament is believed to alter the chalcogenide materialin such a way that the threshold voltage required to form a filamentbetween the load and reference electrodes is reduced. The presence of aconductive filament between the control and another of the terminals mayproduce electric fields or potentials within the chalcogenide materialthat lower the voltage required to form filaments in other portions ofthe chalcogenide material.

As the control signal is raised above the critical value, thecross-section of the filament between the control electrode and anotherof the terminals is believed to increase and a greater volume of thechalcogenide material is believed to be influenced by the controlsignal. The decrease in the threshold voltage between the load andreference electrodes with increasing control voltage may be due to anenlargement of a filament between the control and load or referenceterminal, with the resulting filament boundary approaching the otherterminal (reference or load terminal, respectively. The closer proximitysuggests that a smaller electric field, and hence a smaller thresholdvoltage, is required to establish a filament between the load andreference electrodes.

A filament may form, for example, between the load and referenceelectrodes through a branching of the filament present between thecontrol and reference electrodes. In such a branching process, a portionof a filament between the load and reference electrodes exists withinthe filament present between the control and reference electrodes andthe voltage required to complete the filament may be that required toform a filament between the load electrode and some point, a branchingpoint, along the existing filament. Since the distance between the loadelectrode and a branching point of an existing filament is likely to besmaller than the distance between the load and reference electrodes, themagnitude of the electric field or voltage required to complete afilament to the load electrode is reduced.

In the above example, at some control signal sufficiently above thecritical control signal, it may be expected that the filament formedbetween the control electrode and the reference electrode issufficiently enlarged that the boundary of the filament overlaps ormakes contact with the load electrode. When this occurs, it is believedthat the threshold voltage between the load and reference electrodesdecreases to zero.

In addition to modulating the threshold voltage between two terminals,the instant multi-terminal devices may be used to modulate theconductivity of the chalcogenide material between two terminals. Thiscapability can be demonstrated using the representative device structureillustrated in FIG. 9 and the data plotted in FIG. 10. As an example,consider the application of a voltage of 1.5 V between the load andreference electrodes in the absence of a control voltage. As shown inFIG. 10, application of a voltage of 1.5 V between the load andreference electrodes is unable to switch the device because 1.5 V is asub-threshold voltage. The chalcogenide material between the load andreference electrodes therefore remains in a resistive state and theconductivity between the load and reference electrodes is low. Byapplying a control voltage of sufficient magnitude while maintaining thesame voltage between the load and reference electrodes, it becomespossible to effect a switching event between the load and referenceelectrodes and thereby to induce a pronounced increase in theconductivity of the chalcogenide material between the load and referenceelectrodes.

In the case in which a voltage of 1.5 V is applied between the load andreference electrodes, control voltages between 0 V and 2 V do notdecrease the threshold voltage or influence the conductivity of thechalcogenide material between the load and reference electrodes. Acontrol voltage of 2.5 V, however, decreases the threshold voltage tobelow 1.5 V thereby inducing a transformation of the chalcogenidematerial between the load and reference electrodes from a resistivestate to a conductive state. The transformation is accompanied by adecrease in the voltage between the load and reference electrodes alongwith an increase in current. The voltage between the load and referenceelectrodes decreases to a voltage at or above the holding voltage.

Although the device is resistive and inhibits signal transmissionbetween the load and reference electrodes in the absence of a controlsignal, the device becomes conductive and more readily transmits signalswhen a control voltage of sufficient magnitude is provided. Anappropriate control signal may therefore be used to increase theconductivity of the chalcogenide material between two non-controlelectrodes when a suitable voltage is present therebetween. The suitablevoltage must be less than the threshold voltage with no voltage appliedto the control terminal, but greater than the threshold voltage when theappropriate control signal is applied to the control terminal. Theconductivity of the chalcogenide material between two non-controlelectrodes may analogously be increased by removing or decreasing themagnitude of the control signal applied to the control terminal.Judicious control of the timing, duration and/or magnitude of a controlsignal may thus be used to modulate the conductivity of the chalcogenidematerial between two non-control terminals.

3T OTS devices in accordance with the principles of the presentinvention may exhibit advantageous threshold-voltage modulationcharacteristics. In particular, a 3T OTS device in accordance with theprinciples of the present invention may exhibit different responses tomodulation voltages applied across different terminals. In anillustrative embodiment, for a given voltage applied across a 3T OTS'control and reference terminals, the device's threshold voltage isreduced to a given voltage, or to 0V; but the same voltage appliedacross the device's control and load terminals does not reduce thedevice's threshold voltage. Rather, a larger voltage must be appliedacross the control and load terminals in order to reduce the device'sthreshold voltage.

Similarly, a 3T OTS device may be configured to require a greatermodulation voltage across its control and reference terminals to achievethe same threshold voltage reduction as a smaller modulation voltageapplied across its control and load terminals. As described in greaterdetail in the discussion related to the logic devices of FIGS. 12through 14, such asymmetry (that is, the requirement of a greatercontrol to load terminal (control to reference) voltage than control toreference terminal (control to load) voltage in order to achieve thesame reduction in device threshold voltage is particularly advantageousin logic circuits that employ 3T OTS devices. Typically, in such logiccircuit embodiments, the un-modulated threshold voltage of the 3T OTSdevice is greater than the maximum signal voltage of such a circuit.

The current vs. voltage plot of FIG. 11 depicts the characteristics of a3T OTS device in accordance with the principles of the invention forwhich a modulating voltage is impressed across the control/referenceterminals. In this illustrative embodiment, the device's un-modulatedthreshold voltage is labeled V_(UNMOD). Application of an intermediatemodulation voltage across the control/reference terminals reduces thethreshold voltage to a value V_(INT). Application of a greatermodulation voltage reduces the device's threshold voltage to below thedevice's holding voltage, V_(H). In an illustrative embodiment, theun-modulated threshold voltage V_(UNMOD) is 4.0V, the holding voltage is0.5V, and the control/reference voltage required to reduce the device'sthreshold voltage to below the holding voltage VH is 1.0V. Additionally,in this illustrative embodiment, the modulation voltage across thecontrol/load terminals required to reduce the device's threshold voltageto below V_(H) is 3.0V. Such modulation-voltage asymmetry may beachieved, for example, in the structure of FIG. 9 by locating thecontrol terminal T3 closer to either the reference terminal (to reducethe required control/reference modulation voltage) or to the loadterminal (to reduce the required control/load modulation voltage). Othermeans may be employed to achieve such modulation voltage asymmetry. Forexample, different types of OTS material may be located between thecontrol and reference terminals than located between the control andload terminals. OTS materials between the different terminals may bedifferentially modified by implant, reactive processing, or othertechnique, for example, to achieve the desired threshold-modulatingasymmetry.

The schematic diagrams of FIGS. 12A, 12B, 12C, 12D, and 12E illustratesymbols and associated operating biasing of asymmetrical 3T OTS devices,such as those described in the discussion related to FIG. 11. The symbol3TP of FIG. 12A indicates that the 3T OTS is configured topreferentially operate in a manner analogous to a p-channel field effecttransistor (p-FET), as indicated by the negative signs in front of thecontrol −C and load −L terminals. For this bias configuration, thevoltages associated with the device's terminals, control C, load L andreference R, are measured with respect to the reference terminal R,reference terminal R is assigned a voltage of 0V. In this illustrativeembodiment, the modulating voltage required to reduce the device'sthreshold voltage to a desired value is labeled between the respectiveterminal pairs, with a 1.0 V modulating voltage required between thecontrol and reference terminals and a 3.0V modulating voltage requiredbetween the control and load terminals. As previously noted, the desiredvalue to which the threshold is reduced may be below the device'sholding voltage VH, for example.

FIG. 12B illustrates an equivalent configuration in which the referenceterminal R is at a positive voltage and the load L and control Cterminals may take on values of 0V and above. Such a configuration isthe most commonly encountered circuit configuration.

The “P” in the “3TP” symbol is meant as a short-hand reference to theoperation of an enhancement mode p-channel MOSFET, which the 3TP device,because it is configured to preferentially operate as biased in FIG.12A, to some extent, emulates. The symbol 3TN of FIG. 12C indicates thatthe 3T OTS is configured to preferentially operate, as indicated by thepositive signs in front of the control +C and load +L terminals. Becausethe voltages associated with the device's terminals, control C, load Land reference R, are measured with respect to the reference terminal R,reference terminal R is assigned a voltage of 0V. The “N” in the “3TN”symbol is meant as a short-hand reference to the operation of anenhancement mode n-channel MOSFET, which the 3TN device, because it isconfigured to preferentially operate within a logic circuit in a mannerthat, to some extent, emulates the operation of an N-FET. FIGS. 12D and12E present equivalent symbols for 3TP and 3TN OTS devices,respectively. As may be apparent from the Figures and prior discussion,a single device may be operated as either a 3TN or 3TP device, dependingupon its configuration within a circuit. That is, by reversing the loadand reference connections to an external circuit, the respectivemodulating voltages (R to C and C to L) are reversed, and the deviceoperates accordingly.

Employing the partial-filament model described in the discussion relatedto FIG. 10, it is believed that asymmetric 3T OTS devices in accordancewith the principles of the present invention employ a control voltage toform a filament between the control terminal and either the referenceterminal or load terminal to thereby reduce the overall threshold of thedevice. Depending upon device configuration (e.g., relative spacingbetween control, reference, and load terminals), filament formationbetween one set of terminals requires the application of a greatervoltage than required for the other set of terminals.

Thin film devices, such as 3T OTS devices, may be employed in thin filmlogic circuits in accordance with the principles of the presentinvention to produce standalone thin film memory circuits such as thedecoder described in greater detail in the discussion related to FIG.3A. In an illustrative embodiment, 3T OTS devices are arranged as amulti-phase clock logic circuit to yield thin film decoders, latches,and other circuits that may be employed by a standalone thin film memoryin accordance with the principles of the present invention. Multi-phaseclock logic systems are known and disclosed; for example U.S. Pat. Nos.3,590,273 and 3,935,474, which are hereby incorporated by reference. Amulti-phase logic circuit that employ 3T OTS devices is disclosed in apatent application having publication Ser. No. 11/724,485, filed Mar.15, 2007 entitled, “MULTI-TERMINAL CHALCOGENIDE LOGIC CIRCUITS,” whichis hereby incorporated by reference.

Thin film logic circuits in accordance with the principles of thepresent invention employ thin-film switching devices to executecomplementary logic functions; there is no direct conduction pathbetween a system supply and a system return. In such a system, power isonly dissipated when driving the capacitance associated with terminalsand interconnect to different voltage levels. To a first approximationthere is no static power dissipation. In an illustrative embodiment,complementary logic circuits in accordance with the principles of thepresent invention employ three-terminal threshold switches as switchingelements. More particularly, asymmetric threshold three-terminal ovonicthreshold switches may be combined in accordance with the principles ofthe present invention to produce complementary logic families.

Logic circuits in accordance with the principles of the presentinvention include inverters (NOT), AND, OR, NAND, NOR, XOR, and XNORgates. Any logic function may be built using a subset of these basicgates (a complete set of logic functions may be produced, for example,using only NAND and NOT gates). Such gates may be combined to form aflip-flop, a latch, or a memory decoder, for example. By combiningsequential and combinatorial complementary thin film logic devices inaccordance with the principles of the present invention any digitallogic device, including, for example, shift register, memory controller,arithmetic logic unit, sequencer, microprocessor, or microcontroller maybe implemented. Logic gates in accordance with the principles of thepresent invention may also be employed in embedded devices, such asdevices that include both thin-film memory and thin film logic in asingle standalone circuit.

The 3T OTS logic family disclosed in the '485 application employs amulti-phase clocking system which is described in detail therein. FIG.13 is a schematic diagram of a 3T OTS inverter 1300 in accordance withthe principles of the present invention. Biasing circuitry, clockcircuitry and timing diagrams, discussed in detail in the '485application, will not be described herein. In the schematic diagram ofFIG. 13, the inverter 1300 includes 3TP 1302 and 3TN 1304, with theircontrol terminals connected together to form an input IN to the inverter1300, their load terminals connected together to form an output OUT fromthe inverter 1300 and their respective reference terminals connected toCLOCK and GROUND.

In operation, when the clock input is “high” and a “high” input voltageis applied to the input IN, the positive control-to-reference voltage onthe 3TN device 1304 lowers the threshold voltage of the device 1304 andthe 3TN device, with proper biasing from the next logic stage,“thresholds” (that is, becomes highly conductive). The highly conductivepath through the 3TN device 1304 ensures that the output terminal OUTremains at approximately a holding voltage drop above ground. That is,the 3TN device 1304 drains charge from the output terminal OUT so longas enough current is supplied from the subsequent logic stage backthrough the device 1304, to sustain a holding current. This results inthe OUT voltage dropping to a holding voltage Vh. Any leakage currentthat might otherwise charge the output terminal OUT will be drained bythe 3TN device 1304 to maintain an OUT voltage of Vh.

In an illustrative embodiment a 2.5V signal represents a “HIGH” logicvalue, 0.5V is the holding voltage of the 3T OTS devices, a 0.5V signalrepresents a “LOW” logic value, and the clock value is 3.0V. Therespective threshold-modulation values of the devices are 1.0V and 3.0Vfor the control-reference and control-load terminals. In thisillustrative embodiment, the un-modulated threshold value of the devicesis 4.0V. With an input of 2.5V the 3TN device 1304 thresholds and theoutput voltage is 0.5V. The voltage across the device 1302 is,therefore, 2.5V, but the native threshold voltage of the device is 4.0Vand, consequently, the device 1302 will not threshold. Additionally, thecontrol-to-load and control-to-reference voltages for the device 1302are, respectively, 2.0V and 0.5V, neither of which is sufficient tothreshold the device 1302. Analogous observations may be made withrespect to a logic low input of 0.5V.

The schematic diagram of FIG. 14 illustrates a NAND gate 1400 inaccordance with the principles of the present invention. In thisillustrative embodiment 3TP devices 1402 and 1404 have their referenceterminals connected to the clock signal CLK and their load terminalsconnected to the output OUT. 3TP device 1402 has its control inputconnected to a first NAND input INA and 3TP device 1404 has its controlterminal connected to a second NAND input INB. 3TN device 1406 has itsload terminal connected to the output terminal OUT, its control terminalconnected to input INA and its reference terminal connected to the loadterminal of 3TN device 1408. 3TN device 1408 has its control terminalconnected to input INB and its reference terminal connected to ground.

With 3TP (1402 and 1404) and 3TN (1406 and 1408) devices respectivelyarranged in parallel and series, whenever one or both inputs INA or INBis “LOW,” one of the 3TP devices will threshold and pull the output OUT“HIGH.” Because at least one of the series-connected 3TN devices will be“OFF” (that is, non-thresholding), there will be no contention from a3TN device that might otherwise threshold. Only when both inputs INA andINB are “HIGH” will both 3TN devices threshold and neither 3TP devicethreshold. In that case, the output OUT will be pulled low by the 3TNdevices 1406 and 1408. This describes the operation of a NAND gate.

Using the INVERTER of FIG. 13 and NAND gate of FIG. 14, any logicfunction may be implemented, including, for example, the decoderdescribed in the discussion related to FIG. 3A and latch described inthe discussion related to FIG. 3B. Thin film latches in accordance withthe principles of the present invention may be employed to latch row andcolumn addresses such as may be employed by row and column decoders suchas thin film decoders in accordance with the principles of the presentinvention.

A standalone thin film memory in accordance with the principles of thepresent invention may include built in self test (BIST) circuitry thatis implemented in thin film technology, such as the 3T OTS logicdescribed herein. The BIST circuitry may include microcode storage and amicro-sequencer configured to perform device self tests. More elaboratethin-film logic circuits, such as microprocessors and microcontrollersare within the scope of the invention.

FIGS. 15A through 15K will be used to describe illustrative processesfor forming a thin film memory/thin film logic combination in accordancewith the principles of the present invention. FIG. 15L illustrates astack of memory layers that may be formed on top of the layers describedin the discussion related to FIGS. 15A through 15K. For convenience ofillustration and clarity of explanation, process steps and optionalstructures, such as barrier layers or contact layers, for example, maynot be included in this description. Such steps and structures will beknown to those skilled in the art. The term layer may be used herein torefer to a single layer of material or to a layer that includes one ormore sub-layers, such as a layer of thin film logic that may includesub-layers of electrode material, dielectric, and sub-layers ofthreshold-switching material, for example. Deposition steps may becarried out using known thin-film deposition techniques, including butnot limited to, sputtering (PVD), LPCVD, PECVD, or MOCVD, for example.

In FIG. 15A a thin film of terminal material 1502 (also referred toherein as electrode material) is deposited on a layer of substratematerial 1500. The terminal material 1502 may be Ti, TiN, TiW, C, SiC,TIAIN, TiSiN, polycrystalline silicon, TaN, some combination of these,or other suitable conductors or resistive conductors, for example. Invarious illustrative embodiments the substrate material may be asingle-crystalline material or non-single-crystalline material. In anillustrative embodiment, single-crystalline material may, for example,include conventional single-crystal logic, such as complementary metaloxides semiconductor (CMOS) logic. In non-single-crystalline substrateembodiments, the substrate may be an inexpensive material, such asglass, ceramic, or fiberglass, for example.

In FIG. 15B the terminal material 1502 has been patterned to form aterminal T1. The patterning process may include photo-lithographicsteps, such as masking and etching for example. After the patterning ofFIG. 15B a layer of dielectric material 1504 is deposited in the stepdepicted in FIG. 15C. The dielectric material may be silicon dioxide,for example. In the step related to FIG. 15D the dielectric material1504 is planarized. The planarization process may employ chemicalmechanical polishing (CMP) for example.

After the planarization of FIG. 15D, a layer of terminal material 1506is deposited. This layer of terminal material will be used to form aterminal T3, which will form a control terminal in a 3T OTS device inaccordance with the principles of the present invention. The controlterminal T3 is patterned in the step associated with FIG. 15F and alayer of dielectric material 1508 is deposited over the entire structurein the step associated with FIG. 15G. The dielectric material 1508 isplanarized in the step associated with FIG. 15H.

In the step associated with FIG. 15I, a pore 1510 is formed in themultilayer stack extending through the dielectric layer 1508, the T3terminal layer 1506, the dielectric layer 1504, and stopping at the T1terminal layer 1502. A layer of threshold-switching material 1512 (OTS)is deposited in the step associated with FIG. 15J. The thresholdswitching material 1512 fills or coats the sidewalls and bottom of thepore 1510. A layer 1514 of terminal material T2 is deposited in the stepassociated with FIG. 15K. This layer of terminal material may serve aseither the load or reference terminal of a 3T OTS in accordance with theprinciples of the present invention. The terminal material 1514 may bepatterned, using conventional photolithographic and etching techniques,for example, to form the desired structure shown in FIG. 15L.

As previously described, 3T OTS devices in accordance with theprinciples of the present invention may be formed asasymmetric-threshold devices and the threshold asymmetry of thesedevices may be implemented by forming the device with differentthicknesses of threshold switching material between thecontrol/reference terminal pair than between the control/load terminalpair (e.g., between the top of T1/bottom of T3 and between the bottom ofT2/top of T3). Different types of OTS material may be deposited betweenthe terminal pairs to implement such threshold asymmetry.

A layer of 3T OTS devices formed in the manner just described may beinterconnected to form a layer of complementary logic. This layer ofcomplementary logic may include logic functions such aspreviously-described address latches and decoders, for example. One ormore layers of thin film memory, such as illustrated in FIG. 15L may bedeposited on top of this layer of complementary thin film logic.Additional layers of complementary thin film logic may be interspersedwithin the stack of thin film memory described in greater detail in thediscussion related to FIG. 15L.

Referring to FIG. 15L, a standalone all thin film memory 1500 inaccordance with the principles of the present invention may include thinfilm memory devices and thin film access devices, such as 3T OTSdevices, stacked one upon another. Because thin film materials andprocesses are employed, the memory 1500 may be deposited on a relativelyinexpensive substrate 1502 such as glass, ceramic, or other material; aconventional single crystal silicon substrate is not needed. Thin filmprocessing techniques also permit the stacking of such devices.

The phase change memory elements 14 a-14 d in FIG. 15 of thisillustrative embodiment may be implemented by a series of layersprovided underneath the selection devices 16 a-16 d. The memory elements14 a-14 d and selection devices 16 a-16 d may be, respectively, phasechange memories and 3T OTS devices as described in the discussionrelated to FIGS. 1 and 2. Vias 18 may be provided between adjacentlevels to provide interconnection between wiring conductors on thoselevels.

In this illustrative stacked thin film memory, addressing lines areshared. For example, line 12 d, is shared between directly overlyingcells (including the element 14 d and the selection device 16 d),providing a rowline to those cells; and directly underlying cells(including the selection device 16 c and the memory element 14 c),providing a column line to those cells. Similarly, the line 12 cfunctions for selection of the directly overlying cells (including theselection device 16 c and the memory element 14 c), providing a rowlineto those cells; as well as for selection of the memory cells directlyunderlying line 12 c including memory element 14 b and selection device16 b, providing a column line to those cells.

Stacking of thin film memory devices and shared addressing lines areknown and described in published patent application 2006/0120136entitled, “SHARED ADDRESSE LINES FOR CROSSPOINT MEMORY,” and in U.S.Pat. No. 6,795,338, entitled “Memory Having Access Devices Using PhaseChange Material Such As Chalcogenide,” which are hereby incorporated byreference.

The formation of each layer, such as metal layer N, typically requiresseveral process steps. Such process steps may include: deposition,lithography, etching, cleaning, dielectric deposition, andplanarization, for example. Conventional memories, which rely uponsingle-crystal circuitry to perform memory access functions such asaddress-decoding, have had to rely upon dramatically increased numbersof mask steps (along with ancillary steps just described) in order toachieve greater memory densities. A conventional single-crystal memoryof 128 Mbit or more requires at least thirty mask steps. Each additionalmask step adds significantly to the cost of production, limitsproduction flexibility, decreases reliability, increases productioncycletimes, and increases inventory costs. Because all access devicesand access circuitry, such as address decoding circuitry, is contiguouswith the substrate in a single-crystal memory device, additional metallayers (and associated mask and ancillary steps) may be required toroute address and data lines to memory cells. As features sizes shrink,conventional single-crystal circuitry, including memory devices andperipheral memory circuitry must include isolation features, such asimplant regions. Such isolation regions require additional process stepsincluding: mask, trench etch, clean, implant, clean, dielectric fill,and planarization.

Memory cells in a standalone all-thin-film memory in accordance with theprinciples of the present invention are self-isolating (beingsurrounded, for example, by insulator material such as SiO2) and,therefore, avoid the additional process steps required of conventionalsingle-crystal memory and hybrid memories that employ single-crystalcircuits, such as decoders and select devices, in combination with thinfilm memory cells. By eliminating conventional single-crystal devices astandalone thin film memory in accordance with the principles of thepresent invention eliminates the multiple mask steps associated with theformation of n-channel MOSFETs, with the formation of p-channel MOSFETs,with the isolation of n-channel MOSFETS, with the isolation of P-channelMOSFETS, and with the interconnection to these devices. Additionally, byoff-loading a substantial amount of memory support functions to acontroller, the complexity of a standalone thin film memory inaccordance with the principles of the present invention may besubstantially less than a conventional memory of the same capacity.

In an illustrative embodiment, a standalone thin film memory inaccordance with the principles of the present invention includes only athin film memory array, thin film access devices, thin film decoding,and thin film drivers for input/output operations; all other functionsare executed by a controller adapted to controlling one or morestandalone thin film memories in accordance with the principles of thepresent invention. By eliminating the complexity associated with amemory that includes peripheral support circuits, along with a memoryarray and access circuits, a standalone thin film memory in accordancewith the principles of the present invention avoids the use ofadditional metal layers (for example, three to five metal layers, eachrequiring two to four mask steps) often required to interconnectconventional, complex, single-crystal memories.

Even hybrid thin-film/single-crystal memories, which employ asingle-crystal substrate that includes single-crystal circuitry such asaddress-decoding circuitry, may be forced to use more mask steps inorder to route address and data lines to memory cells.

A standalone all-thin-film-memory in accordance with the principles ofthe present invention greatly reduces the number of mask steps requiredfor each complete memory layer of 128 Mbit or more. In some embodiments,an all-thin-film memory in accordance with the principles of the presentinvention may reduce the number of mask steps required for the formationof a complete memory layer of 128 Mbit or more to a range of betweeneight and twenty mask steps. By sharing interconnect layers, as in thestacked embodiment of FIG. 15, the number of layer, and associated masksteps per layer of memory, can be reduced even further.

In the illustrative embodiment, in FIG. 16 a memory system 1600 inaccordance with the principles of the present invention includes one ormore standalone thin film memories 1602 deposited on a non-crystallinesubstrate 1604. Such a non-crystalline substrate 1604 may be a surfacemount PC board, for example. A plurality of standalone memory devices1602 in accordance with the principles of the present invention may bejoined via interconnecting conductive lines 1606 patterned on thesubstrate. Conductive interconnecting lines may be patterned in amultilayer substrate, in order to connect components through conductivelines situated on different layers of the substrate, for example.

A memory controller 1605 may be formed on the same substrate 1604 andconfigured to operate the standalone memories 1602 in a mannerpreviously described. The memory controller 1605 may itself be athin-film component implemented using thin film logic such as the 3T OTSlogic described in the discussion related to FIGS. 7-14. Alternatively,the memory controller may be formed in separate processes, such as aconventional CMOS process, then connected to the standalone memoriesformed on the non-crystalline substrate through processes such as thoseemployed in hybrid circuit manufacture, for example. The memory system1600 may communicate with other components using conventionalinterconnection components, such as edge connector 1608 or otherself-contained connector 1610 which may be a high speed optical orcoaxial connector, for example.

The thin film electronic device(s) described in the discussion relatedto the previous figures may be employed to particular advantage in awide variety of systems. The schematic diagram of FIG. 17 will bediscussed to illustrate the devices' use in a few such systems. Theschematic diagram of FIG. 17 includes many components and devices, someof which may be used for specific embodiments of a system in accordancewith the principles of the present invention and while others not used.In other embodiments, other similar systems, components and devices maybe employed. In general, the system includes logic circuitry configuredto operate along with standalone thin film memory which may includephase change memory. The logic circuitry may be discrete, programmable,application-specific, or in the form of a microprocessor,microcontroller, or digital signal processor, for example. In someembodiments, the logic circuitry may be implemented using thin filmlogic. And the embodiments herein may also be employed on integratedchips or connected to such circuitry. The exemplary system of FIG. 17 isfor descriptive purposes only. Although the description may refer toterms commonly used in describing particular computer, communications,tracking, and entertainment systems; the description and conceptsequally apply to other systems, including systems having architecturesdissimilar to that illustrated in FIG. 17. The electronic system 1700,in various embodiments, may be implemented as, for example, a generalpurpose computer, a router, a large-scale data storage system, aportable computer, a personal digital assistant, a cellular telephone,an electronic entertainment device, such as a music or video playbackdevice or electronic game, a microprocessor, a microcontroller, adigital signal processor, or a radio frequency identification device.Any or all of the components depicted in FIG. 17 may employ a standalonethin film memory or a chalcogenide electronic device, such as achalcogenide-based nonvolatile memory and/or threshold switch, forexample.

In an illustrative embodiment, the system 1700 may include a centralprocessing unit (CPU) 1705, which may be implemented with some or all ofa microprocessor, a random access memory (RAM) 1710 for temporarystorage of information, and a read only memory (ROM) 1715 for permanentstorage of information. A memory controller 1720 is provided forcontrolling RAM 1710. In accordance with the principles of the presentinvention, all of, or any portion of, any of the memory elements (e.g.RAM or ROM) may be implemented as a standalone thin film memory whichmay include chalcogenide-based nonvolatile memory.

An electronic system 1700 in accordance with the principles of thepresent invention may be a microprocessor that operates as a CPU 1705,in combination with embedded chalcogenide-based electronic nonvolatilememory that operates as RAM 1710 and/or ROM 1715, or as a portionthereof. In this illustrative example, themicroprocessor/chalcogenide-nonvolatile memory combination may bestandalone, or may operate with other components, such as those of FIG.17 yet-to-be described.

In implementations within the scope of the invention, a bus 1730interconnects the components of the system 1700. A bus controller 1725is provided for controlling bus 1730. An interrupt controller 1735 mayor may not be used for receiving and processing various interruptsignals from the system components. Such components as the bus 1730, buscontroller 1725, and interrupt controller 1735 may be employed in alarge-scale implementation of a system in accordance with the principlesof the present invention, such as that of a standalone computer, arouter, a portable computer, or a data storage system, for example.

Mass storage may be provided by diskette 1742, CD ROM 1747, or harddrive 1752. Data and software may be exchanged with the system 1700 viaremovable media such as diskette 1742 and CD ROM 1747. Diskette 1742 isinsertable into diskette drive 1741 which is, in turn, connected to bus1730 by a controller 1740. Similarly, CD ROM 1747 is insertable into CDROM drive 1746 which is, in turn, connected to bus 1730 by controller1745. Hard disc 1752 is part of a fixed disc drive 1751 which isconnected to bus 1730 by controller 1750. Although conventional termsfor storage devices (e.g., diskette) are being employed in thisdescription of a system in accordance with the principles of the presentinvention, any or all of the storage devices may be implemented usingstandalone thin film memory which may include chalcogenide-basednonvolatile memory in accordance with the principles of the presentinvention. Removable storage may be provided by a nonvolatile storagecomponent, such as a thumb drive, that employs a chalcogenide-basednonvolatile memory in accordance with the principles of the presentinvention as the storage medium. Storage systems that employchalcogenide-based nonvolatile memory as “plug and play” substitutes forconventional removable memory, such as disks or CD ROMs or thumb drives,for example, may emulate existing controllers to provide a transparentinterface for controllers such as controllers 1740, 1745, and 1750, forexample.

User input to the system 1700 may be provided by any of a number ofdevices. For example, a keyboard 1756 and mouse 1757 are connected tobus 1730 by controller 1755. An audio transducer 1796, which may act asboth a microphone and/or a speaker, is connected to bus 1730 by audiocontroller 1797, as illustrated. Other input devices, such as a penand/or tabloid may be connected to bus 1730 and an appropriatecontroller and software, as required, for use as input devices. DMAcontroller 1760 is provided for performing direct memory access to RAM1710, which, as previously described, may be implemented in whole orpart using chalcogenide-based nonvolatile memory devices in accordancewith the principles of the present invention. A visual display isgenerated by video controller 1765 which controls display 1770. Thedisplay 1770 may be of any size or technology appropriate for a givenapplication.

In a cellular telephone or portable entertainment system embodiment, forexample, the display 1770 may include one or more relatively small (e.g.on the order of a few inches per side) LCD displays. In a large-scaledata storage system, the display may be implemented as large-scalemulti-screen, liquid crystal displays (LCDs), or organic light emittingdiodes (OLEDs), including quantum dot OLEDs, for example.

The system 1700 may also include a communications adaptor 1790 whichallows the system to be interconnected to a local area network (LAN) ora wide area network (WAN), schematically illustrated by bus 1791 andnetwork 1795. An input interface 1799 operates in conjunction with aninput device 1793 to permit a user to send information, whether commandand control, data, or other types of information, to the system 1700.The input device and interface may be any of a number of commoninterface devices, such as a joystick, a touch-pad, a touch-screen, aspeech-recognition device, or other known input device. In someembodiments of a system in accordance with the principles of the presentinvention, the adapter 1790 may operate with transceiver 1773 andantenna 1775 to provide wireless communications, for example, incellular telephone, RFID, and wifi computer implementations.

Operation of system 1700 is generally controlled and coordinated byoperating system software. The operating system controls allocation ofsystem resources and performs tasks such as processing scheduling,memory management, networking, and I/O services, among other things. Inparticular, an operating system resident in system memory and running onCPU 1705 coordinates the operation of the other elements of the system1700.

In illustrative handheld electronic device embodiments of a system 1700in accordance with the principles of the present invention, such as acellular telephone, a personal digital assistance, a digital organizer,a laptop computer, a handheld information device, a handheldentertainment device such as a device that plays music and/or video,small-scale input devices, such as keypads, function keys and soft keys,such as are known in the art, may be substituted for the controller1755, keyboard 1756 and mouse 1757, for example. Embodiments with atransmitter, recording capability, etc., may also include a microphoneinput (not shown).

In an illustrative RFID transponder implementation of a system 1700 inaccordance with the principles of the present invention, the antenna1775 may be configured to intercept an interrogation signal from a basestation at a frequency F₁. The intercepted interrogation signal wouldthen be conducted to a tuning circuit (not shown) that accepts signal F₁and rejects all others. The signal then passes to the transceiver 1773,where the modulations of the carrier F₁ comprising the interrogationsignal are detected, amplified and shaped in known fashion. The detectedinterrogation signal then passes to a decoder and logic circuit whichmay be implemented as discrete logic in a low power application, forexample, or as a microprocessor/memory combination as previouslydescribed. The interrogation signal modulations may define a code toeither read data out from or write data into a chalcogenide-basednonvolatile memory in accordance with the principles of the presentinvention. In this illustrative embodiment, data read out from thememory is transferred to the transceiver 1773 as an “answerback” signalon the antenna 1775 at a second carrier frequency F₂. In passive RFIDsystems, power is derived from the interrogating signal and memory suchas provided by a chalcogenide-based nonvolatile memory in accordancewith the principles of the present invention is particularly well suitedto such use.

1. An asymmetric threshold three terminal threshold electronic switchingdevice comprising: a first terminal; a second terminal; a thirdterminal; and a threshold-switching material in electrical communicationwith the first, second, and third terminals, said threshold-switchingmaterial comprising a chalcogenide material, wherein an electricalcharacteristic of the device manifested between the second and thirdterminals is affected by the application of a signal between the firstand second terminals to a different degree than the same characteristicis affected by the application of the signal between the first and thirdterminals.
 2. The electronic switching device of claim 1 wherein theelectrical characteristic is a threshold voltage.
 3. The electronicswitching device of claim 2, wherein the application of a given signallevel between the first and second terminals modulates the thresholdvoltage of the device to a greater extent than the application of thesame signal level between the first and third terminals.
 4. Theelectronic switching device of claim 3 wherein the application of agiven signal level between the first and second terminals reduces thethreshold voltage between the second and third terminals to a minimumvalue and application of the same signal level between the first andthird terminals does not reduce the threshold voltage between the secondand third terminals to a minimum value.
 5. The electronic switchingdevice of claim 2 wherein a first modulating voltage applied between thefirst and second terminals which is the minimum required to reduce thethreshold voltage between the second and third terminals to a minimum; asecond modulating voltage applied between the first and third terminalswhich is the minimum required to reduce the threshold voltage betweenthe second and third terminals to a minimum; and the threshold voltagebetween the second and third terminals with no modulating voltageapplied is equal to the total of the first and second modulatingvoltages.
 6. The electronic switching device of claim 2 wherein a firstmodulating voltage applied between the first and second terminals whichis the minimum required to reduce the threshold voltage between thesecond and third terminals to a minimum; a second modulating voltageapplied between the first and third terminals which is the minimumrequired to reduce the threshold voltage between the second and thirdterminals to a minimum; and the threshold voltage between the second andthird terminals with no modulating voltage applied is greater than thetotal of the first and second modulating voltages.
 7. The electronicswitching device of claim 2 wherein a first modulating voltage appliedbetween the first and second terminals which is the minimum required toreduce the threshold voltage between the second and third terminals to aminimum; a second modulating voltage applied between the first and thirdterminals which is the minimum required to reduce the threshold voltagebetween the second and third terminals to a minimum; and the thresholdvoltage between the second and third terminals with no modulatingvoltage applied is less than the total of the first and secondmodulating voltages.
 8. The electronic switching device of claim 1wherein the electrical characteristic is the conductivity of thethreshold-switching material.
 9. The electronic device of claim 1,wherein the chalcogenide material comprises an amorphous phase.
 10. Anasymmetric threshold three terminal threshold electronic devicecomprising: a first terminal; a second terminal; a third terminal; and athreshold-switching material in electrical communication with the first,second, and third terminals, wherein the thickness of the thresholdswitching material between the first and second terminals is not equalto the thickness of the threshold switching material between the firstand third terminals.
 11. The electronic device of claim 10, wherein thethreshold-switching material comprises a chalcogenide material.
 12. Theelectronic device of claim 11, wherein the chalcogenide materialcomprises an amorphous phase.
 13. The electronic device of claim 10wherein the thickness of threshold switching material between the firstand second terminals is greater than the thickness of thresholdswitching materials between the first and third terminals.
 14. Theelectronic device of claim 13 wherein an electrical characteristic ofthe device manifested between the second and third terminals is affectedby the application of a signal between the first and second terminalsdifferently than the same characteristic is affected by the applicationof a signal between the first and third terminals.
 15. The electronicdevice of claim 14 wherein the threshold voltage between the second andthird electrodes is modulated to a greater extent by application of amodulating voltage applied between the first and third terminals than byapplication of a modulating voltage applied between the first and secondterminals.
 16. The electronic device of claim 10 wherein the compositionof the threshold switching material between the first and secondterminals is different from the composition of the threshold switchingmaterial between the first and third terminals.
 17. An asymmetricthreshold three terminal threshold electronic device comprising: a firstterminal; a second terminal; a third terminal; and a threshold-switchingmaterial in electrical communication with the first, second, and thirdterminals, wherein the composition of threshold switching materialbetween the first and second terminals is different from the compositionof threshold material between the first and third terminals.
 18. Theelectronic device of claim 17, wherein the threshold-switching materialcomprises a chalcogenide material.
 19. The electronic device of claim 18wherein the threshold voltage between the second and third electrodes ismodulated to a greater extent by application of a modulating voltageapplied between the first and third terminals than by application of amodulating voltage applied between the first and second terminals. 20.The electronic device of claim 17 wherein the composition of thethreshold switching material between the first and second terminalsexhibits a higher threshold voltage than the composition of the materialbetween the first and third terminals.
 21. The electronic device ofclaim 20, wherein the composition of the threshold switching materialbetween the first and second terminals comprises a sulfur-richchalcogenide material.
 22. The electronic device of claim 20, whereinthe composition of the threshold switching material between the firstand second terminals comprises a selenium-rich chalcogenide material.23. The electronic device of claim 20, wherein the composition of thethreshold switching material between the first and third terminalscomprises a tellurium-rich chalcogenide material.
 24. The electronicdevice of claim 17 wherein the electronic device is a thin-film device.25. The electronic device of claim 18, wherein the chalcogenide materialcomprises an amorphous phase.
 26. An asymmetric threshold three-terminalthreshold electronic device comprising: a first terminal; a secondterminal; a third terminal; and a threshold-switching material inelectrical communication with the first, second, and third terminals,wherein the threshold voltage between the second and third terminals isreduced by the application of a voltage difference between the first andsecond terminals to a greater extent than by the application of the samevoltage difference between the first and third terminals, the thirdterminal configured for coupling to a circuit load or output terminal,the second terminal configured for coupling to a circuit supply, and thefirst terminal configured for coupling to an input signal.
 27. Theelectronic device of claim 26 wherein the electronic device is anasymmetric-threshold three-terminal ovonic threshold switch.
 28. Theelectronic device of claim 27 wherein the third terminal is coupled toground through another electronic device.
 29. The electronic device ofclaim 28 wherein the third terminal is coupled to ground through asecond asymmetric-threshold three-terminal threshold electronic device.30. The electronic device of claim 29 wherein the secondasymmetric-threshold three-terminal threshold electronic device is anasymmetric-threshold three-terminal ovonic threshold switch.
 31. Theelectronic device of claim 30, wherein the second asymmetric thresholdthree-terminal threshold electronic device includes: a first terminal; asecond terminal; a third terminal; and a threshold-switching material inelectrical communication with the first, second, and third terminals,wherein the threshold voltage between the second and third terminals isreduced by the application of a voltage difference between the first andsecond terminals to a greater extent than by the application of the samevoltage difference between the first and third terminals, the thirdterminal configured for coupling to a circuit load or output terminal,the second terminal configured for coupling to a circuit ground, and thefirst terminal configured for coupling to an input signal.
 32. Theelectronic device of claim 31 wherein the third terminal of the secondasymmetric threshold three-terminal threshold electronic device isconfigured for coupling to a circuit supply through the first asymmetricthreshold three-terminal threshold electronic device.
 33. The electronicdevice of claim 32 wherein the first and second asymmetric thresholdthree-terminal threshold electronic devices are configured as aninverter.
 34. The electronic device of claim 33 comprising a pluralityof asymmetric threshold three-terminal threshold electronic devices,wherein the asymmetric three-terminal electronic devices are configuredas a NAND gate.
 35. The electronic device of claim 33 comprising aplurality of asymmetric threshold three-terminal threshold electronicdevices, wherein the asymmetric three-terminal electronic devices areconfigured as a decoder.
 36. The electronic device of claim 33comprising a plurality of asymmetric threshold three-terminal thresholdelectronic devices, wherein the asymmetric three-terminal electronicdevices are configured as a latch.
 37. The electronic device of claim 33comprising a plurality of asymmetric threshold three-terminal thresholdelectronic devices, wherein the asymmetric three-terminal electronicdevices are configured as a flip-flop.
 38. A memory, comprising: aplurality of memory cells; and an asymmetric threshold three-terminalthreshold decoder, the decoder comprising a chalcogenide material andconfigured to decode addresses associated with the memory cells.
 39. Anelectronic device, comprising: a plurality of memory cells; anasymmetric threshold three-terminal decoder, the decoder comprising achalcogenide material and configured to decode addresses associated withthe memory cells; and a microprocessor configured to access theplurality of memory cells through the asymmetric thresholdthree-terminal decoder.
 40. The electronic device of claim 39 whereinthe decoder comprises asymmetric threshold ovonic threshold switchdevices.
 41. The electronic device of claim 40 further comprising atransmitter/receiver configured to transmit data from and receive datafor the microprocessor or standalone thin film memory.
 42. Theelectronic device of claim 41 wherein the memory, microprocessor andtransmitter/receiver are configured as a cellular telephone.
 43. Theelectronic device of claim 40 wherein the memory and microprocessor areconfigured as a handheld entertainment device.
 44. The electronic deviceof claim 40 wherein the memory and microprocessor are configured as ageneral purpose computer.
 45. The electronic device of claim 40 whereinthe memory is configured as a solid state drive.